1. Field of the Invention
The present invention relates to a magnetooptical recording medium.
2. Related Background Art
In recent years, many efforts have been made to develop an optical recording/reproduction method which can satisfy various requirements including high density, large capacity, high access speed, and high recording/reproduction speed, and a recording apparatus, a reproduction apparatus, and a recording medium which are used for this method.
Of various optical recording/reproduction methods, a magnetooptical recording/reproduction method is most attractive due to its unique advantages that information can be erased after it is recorded, and new information can be repetitively recorded.
A recording medium used in this magnetooptical recording/reproduction method uses a magnetic thin film having a perpendicular magnetic anisotropy as a recording layer. The magnetic thin film comprises an amorphous heavy rare earth-transition metal alloy. A typical example of the alloy includes GdFe, FdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like.
In general, the direction of magnetization of a recording layer is initialized in one of upward and downward directions before recording. In recording, the temperature of a portion of the recording layer is increased by a laser beam focused to have a spot size of about 1 .mu.m to a temperature near the Curie temperature, and a mark having magnetization in the opposite direction is formed using a recording field Hb. In general, information is expressed by the presence/absence or the recorded mark length of a recorded mark. Information is reproduced using polarization of reflected light of the radiated laser beam at a Kerr rotation angle .theta.k. When an erase field He is applied in a direction of magnetization opposite to that in the recording mode, and the temperature of a corresponding portion of the recording layer is increased by a laser beam to a temperature near the Curie temperature, the formed mark can be erased. Information can be repetitively rewritten by repeating erasing and recording operations.
It is generally difficult to obtain, using a single magnetic material, a recording layer which has a low Curie temperature to allow easy recording, has a large coercivity to allow high preservation stability of recorded information, and has a large .theta.k and a high C/N ratio in the reproduction mode.
For this reason, a two-layered film magnetooptical recording medium has been proposed. In this medium, functions required for the recording layer are divided into two layers, e.g., a recording layer (or a recording holding layer) in the strict sense of the word and a reproduction layer, and magnetic materials having compositions respectively suitable for these layers are selected. As one of recording media of this type, a recording medium disclosed in Japanese Laid-Open Patent Application No. 57-78652 is known. This recording medium comprises a two-layered film including a magnetic layer (lower coercivity layer) having a relatively high Curie temperature and a relatively low coercivity at room temperature, and a magnetic layer (higher coercivity layer) having a relatively low Curie temperature and a relatively high coercivity at room temperature. The higher and lower coercivity layers are exchange-coupled to each other. This recording medium has high recording sensitivity since recording is performed at a temperature near the lower Curie temperature of the higher coercivity layer, and has a high C/N ratio since reproduction of recorded information is performed from the side of the lower coercivity layer which has a high Curie temperature and a large Kerr rotation angle. Also, this recording medium has excellent preservation performance since information is preserved by the higher coercivity layer.
Japanese Laid-Open Patent Application No. 63-14342 describes a magnetooptical recording medium which has a higher coercivity layer exhibiting a low Curie temperature, and a lower coercivity layer exhibiting a high Curie temperature, and in which each of these layers comprises an amorphous alloy consisting of a rare earth element and a transition metal element. The film thickness of the higher coercivity layer is 50 .ANG. or more, and is smaller than that of the lower coercivity layer. Japanese Laid-Open Patent Application No. 59-168954 also describes a magnetooptical recording medium having a similar arrangement, and proposes a magnetooptical recording medium which allows writing of information with lower energy, and assures a high S/N ratio due to a large Kerr rotation angle in the reproduction mode.
In general, a recording layer in a magnetooptical recording medium is formed on a substrate on which a guide groove is formed in advance for the purpose of tracking of a laser beam. However, the above-mentioned recording medium, in which the recording layer comprising the two-layered film is formed on the substrate pre-formed with the guide groove, has poor erase characteristics of a recorded mark. For example, in order to completely erase a recorded mark, radiation of laser power 1.1 times or more of that radiated in the recording mode is required. Normally, in a magnetooptical recording medium having a single recording layer, a recorded mark can be almost completely erased by radiating a laser beam with power substantially equal to that radiated in the recording mode. As compared to this medium, the above-mentioned recording medium is a medium in which information is not easy to erase.
A semiconductor laser which is currently used in a magnetooptical recording apparatus has a limited laser power output. For this reason, in a recording medium in which information can be erased by only a semiconductor laser with high-intensity laser power, a recorded mark cannot often be erased due to the performance of the apparatus. In order to surely erase information, the linear velocity of the recording medium may be lowered. In this case, however, the merits, i.e., recording at a high linear velocity due to high recording sensitivity, and a high information transfer rate, of the two-layered film magnetooptical recording medium are lost.